Low molecular weight guanosine triphosphate (GTP)/guanosine diphosphate (GDP) binding GTPases are signal transducers that play a central role in many important cellular functions including cytoskeleton organization, proliferation, differentiation, and development (Khosravi-Far, R. and Der, C. J., Cancer Metastasis Rev, 1994, 13:67–89; Zohn, I. M. et al., Oncogene, 1998, 17:1415–38). Some of these small G-proteins have also been involved in pathological conditions such as malignant transformation. For example, Ras, RhoA, Rac1 and cdc42 have been implicated in oncogenesis, invasion and metastasis (Hall, A., Science, 1998, 279:509–14; Khosravi-Far, R. and Der, C. J., Cancer Metastasis Rev, 1994, 13:67–89; Pruitt, K. and Der, C. J., Cancer Lett, 2001, 171:1–10; Symons, M. and Settleman, J., Trends Cell Biol, 2000, 10:415–9; Van Aelst, L. and D'Souza-Schorey, C., Genes Dev, 1997, 11:2295–322). In contrast, a closely related family member, RhoB, which shares 86% amino acid sequence identity with RhoA, has recently been shown to have tumor suppressive activity (Chen, Z. et al., J Biol Chem, 2000, 275:17974–8; Du, W. et al., Mol Cell Biol, 1999, 19:1831–40; Du, W. and Prendergast, G. C., Cancer Res, 1999, 59:5492–6).
Though highly homologous, RhoA and RhoB are distinct in several aspects. While RhoA is constitutively expressed, RhoB is inducible and has been characterized as an early-response gene. RhoB is known to be induced by DNA damaging agents such as UV and γ-irradiation, N-methyl-N-nitrosourea and cisplatin as well as other chemicals such as methyl methansulfonate, hydrogen peroxide (H2O2) and cycloheximide, and growth factors such as EGF and PDGF (Fritz, G. and Kaina, B., J Biol Chem, 1997, 272:30637–44; Fritz, G. et al., J Biol Chem, 1995, 270:25172–7; Jahner, D. and Hunter, T., Mol Cell Biol, 1991, 11:3682–90). Furthermore, unlike RhoA that has a half life of 24 hours, RhoB's half life is much shorter and is on the order of 1–2 hours (Lebowitz, P. F. et al., Mol Cell Biol, 1995, 15:6613–22). Finally, while RhoA mediates oncogenesis, RhoB has tumor suppressive activity, and it has been shown to inhibit tumor cell proliferation in rodent fibroblasts as well as human cancer cells (Chen, Z. et al., J Biol Chem, 2000, 275:17974–8; Du, W. et al., Mol Cell Biol, 1999, 19:1831–40; Du, W. and Prendergast, G. C., Cancer Res, 1999, 59:5492–6). RhoB also inhibits oncogenic and tumor survival pathways and induces apoptosis in vitro and inhibits the growth of human tumors grown in nude mice (Chen, Z. et al., J Biol Chem, 2000, 275:17974–8). Furthermore, targeted deletion of RhoB results in mice that are more sensitive to skin carcinogenesis (Liu, A. et al., Mol Cell Biol, 2000, 20:6105–13). Ras-transformed RhoB (−/−) fibroblasts from these mice are resistant to γ-irradiation-, doxorubicin-, and TAXOL-induced apoptosis (Liu, A. et al., Proc Natl Acad Sci USA, 2001, 98:6192–7). Finally, RhoB expression levels have been recently shown to dramatically decrease with the aggressiveness of tumors in head, neck and brain cancer patients (Adnane, J. et al., Clin Cancer Res, 2002, 8:2225–32; Forget, M. A. et al., Clin Exp Metastasis, 2002, 19:9–15).